The present invention relates to a method of detecting foreign matters mixed in a liquid contained in a transparent receptacle, coupled with an apparatus relevant thereto. To be more precise, it relates to a method of detecting undesirable, minute foreign matters that might be present within an air-tight transparent receptacle such as ampules filled with liquid medicine, as well as an apparatus relevant thereto.
Heretofore, as an art of this kind for the purpose of detecting foreign matters, there is known a method employing an apparatus which is devised as follows: transparent receptacles filled with a liquid are placed at regular intervals on the periphery of a turntable which rotates continuously as elucidated later on, and these receptacles are brought to a sudden stop after turning at high speed on the turntable during its rotation, whereby foreign matters mixed in the liquid are made to float; rays of light are applied to each of the thus stopped receptacles at almost a right angle by means of a projector which is installed outside the receptacle and repeats reciprocating motion of rapidly returning after moving by a fixed distance synchronously with the receptacle, and the flux of light transmitted to the outside of the receptacle is received by the ray-receiving surface of a ray receiver which is installed on the opposite side of the projector relative to the receptacle and moves synchronously with the projector; thus, when there occurs any decrease in the amount of light in respect of the image formed on the ray-receiving surface due to interruption of the light by foreign matters in the course of its transmission through the liquid, it is to be detected and the presence of foreign matters is thereby detected.
In an apparatus of this type, the projector is provided with a condenser lens for projecting parallel rays to the receptacle, while the ray receiver is provided with a focusing lens for focusing that parallel rays transmitted through the receptacle. As a focusing lens for this purpose, a lens set for use in ordinary cameras as lens system that displays a sufficient resolution even on minute foreign matters having a particle size of 50.mu. or thereabouts has been used. In this context, said focusing lens is required to be capable of not only forming a distinct image of minute foreign matters on the ray-receiving area but catching the whole flux of light given out from a light source and transmitted through the receptacle to make the brightness of the background as high and uniform as possible in order to improve the inspecting sensitivity, and therefore it has hitherto been unavoidable to employ a focusing lens having an aperture larger than the depth of the liquid within the receptacle.
As a consequence, the prior art is defective in that, though it is possible to perform the inspection by the use of an ordinary lens set of about 40 mm in aperture having a focal distance of, for instance, f55/F1.4 in the case of ampules of 2 ml or 5 ml, it is extremely difficult to materialize a focusing lens having a sufficient resolution on minute foreign matters and an adequate aperture for use in inspecting a receptacle wherein the depth of the liquid is 100 mm or more, such as 500 ml liquid supply bottle.
Besides, the prior art is defective in that, employment of a focusing lens having a large aperture necessitates provision of a considerably wide space between the focusing lens and the ray-receiving surface, entailing requirement for enlargement of the ray receiver as a whole that is not only difficult to design but apt to cause oscillation of the apparatus at the time of reciprocating motion during the inspection and bring about blurring of the resulting image. To be more precise, in the case where a lens having a focal distance of 135 mm, for instance, is employed, in order to obtain an equimultiplied image, it is necessary to make the distance between the receptacle and the lens and that between the lens and the ray-receiving surface twice longer than the foregoing focal distance, or 270 mm. Accordingly, the distance between the receptacle and the ray-receiving surface is required to be 540 mm and the size of the resulting ray receiver becomes large, entailing the foregoing defects.
Moreover, in the case of a transparent receptacle whose bottom makes a right angle with the side wall thereof (for example through a curved corner portion as shown in FIG. 5 herein), when parallel rays are projected to the receptacle at almost a right angle as in the foregoing, the incident rays are refracted at a large angle by the bottom of receptacle to hamper the transmission of flux of light through the liquid within the bottom portion, entailing a defect that foreign matters which might be present in said portion can not be detected.